This invention relates to an image reader such as a color image scanner.
As shown in FIG. 1, in conventional image scanners, like cylindrical type scanners, an original image to be read, for example, a film image 2, is wound around a transparent cylinder 4 which rotates about a rotation axis 3 (in the primary scanning direction). An inner light source (not shown) for illuminating a small area of the film image 2 is located inside the cylinder 4. A scanning head 6 is located outside the cylinder 4. The scanning head 6 comprises a pickup lens 8, a half mirror 10, a main aperture board set 12, a sub-aperture board set 14, dichroic mirrors, 16, 18, and 20, for red (R), green (G) and blue (B) respectively, filters, 22, 24, and 26 for R, G, and B respectively, a filter 28 for an unsharp signal (for G filter, for example), photomultipliers 30, 32, 34, and 36 working as a photoelectric transfer device for providing electric signals responsive to the quantity of received lights, and an outer light source (not shown). The scanning head 6, integrated with the inner light source (not shown), shifts gradually in the direction of the rotation axis 3 (in a secondary scanning direction) in synchrony with the rotation of the cylinder 4. While scanning the film image 2 in both the primary scanning direction 42 and the secondary scanning direction 44, the pickup lens 8 forms an image of a small area of the original film image 2 onto both the main aperture board set 12 and the sub-aperture board set 14.
As shown in FIG. 2A and FIG. 2B, the main aperture board set 12 has a transmission part 46 which forms from a square or round opening. The diameter of the transmission part 46 is normally kept small for better reading resolution. The diameter of the transmission part 46 is adjusted, depending on an enlargement ratio (usually within a range from 20% to 2000%) employed in reproducing the original image. The smaller the enlargement ratio being used, the larger the diameter needs to be set. Conversely, the larger the enlargement ratio, the smaller the diameter is set.
Again referring to FIG. 1, lights from the pickup lens 8, reflected by the half mirror 10, are directed to the transmission part 46, which allows the lights corresponding to the center part of a small area of the original image 2 to pass through. The lights are then separately directed to photomultipliers 30, 32, and 34, after being separated into three colors by means of dichroic mirrors 16, 18 and 20, and filters 22, 24, and 26. The sub-aperture board set 14 and the main aperture board set 12 have an equal-location relationship; for example, the distance between the sub-aperture board set 14 and the half mirror 10 is nearly identical to the distance between the main aperture board set 12 and the half mirror 10. As shown in FIG. 3A and FIG. 3B, the sub-aperture board set 14 comprises a transmission part 48, which has a square or round opening, the size of which is larger than that of the transmission opening 46 on the main aperture board set 12. The function of the transmission part 48 is to enhance the outline of shapes in the original image.
Again referring to FIG. 1, the lights, after passing through the half mirror 10, are directed onto the transmission part 48, which allows lights corresponding to the wide part, thereby including the above-mentioned center part, of a small area of the original image to pass through. After passing through the transmission part 48, the lights corresponding to the wide part, are directed to the photomultiplier 36 via a filter 28. The output of the photomultipliers 30, 32 and 34 is directly related to the input level of the R, G and B components. The output signals are separately amplified in a logarithmic manner by the logarithmic amplifiers 50, 52 and 54 respectively. Assuming that an original image, such as the dark-light pattern indicated in FIG. 4A, is scanned in the primary scanning direction 42, the high-resolution sharp signals, SR, SG, and SB for the R, G, and B components, are obtained as shown in FIG. 4B. (Note that FIG. 4B shows only the sharp signal SG.)
The sharp signal SG is generally used by itself, for reasons of convenience, as the sole outline enhancing signal, on the assumption that it represents the other sharp signals as well.
Occasionally, an outline enhancing signal is also used which may be obtained by weight averaging the three sharp signals for R, G, and B.
Further, assuming that the original dark-light pattern image indicated in FIG. 4A is scanned in the primary scanning direction 42, the low-resolution unsharp signal U is obtained as shown in FIG. 4C. In FIG. 1, an operational amplifier 58 subtracts the unsharp signal U from the sharp signal SG. A potentiometer 60 adjusts the level, or multiplies the resulting signal (SG-U), by an adjusting factor K, thus putting out (k(SG-U)) as the outline enhancing signal. FIG. 4D indicates that the level of the resulting outline enhancing signal (k(SG-U)) varies largely in the vicinities of a and b in FIG. 4A, where a dark area and a light area meet. The adders 62, 64, and 66 add the outline enhancing signal (k(SG-U)) to SR, SG, and SB respectively as in FIG. 1. FIG. 4E is an example showing the outline enhanced signal (SG+k(SG-U)). With the cylinder 4 rotating, a clock pulse is provided. Synchronized to the clock pulse, the analog-to-digital (A/D) converters 68, 70, and 72 convert the outline enhanced signals from analog to digital form. The pitch of the primary scanning and secondary-scanning directions needs to be adjusted in accordance with the enlargement ratio desired. Note that the pulse interval of the clock pulse determines the pitch of the main scanning.
As mentioned above, conventional image readers allow the lights corresponding to the center part of a small area of the original image to pass through the transmission part of the main aperture board set. High-resolution and excellent-quality sharp signals thus result.
However, conventional image readers have the disadvantage that, because of their high reading resolution, they also read graininess (magnified by the pickup lens) of a size greater than, or even nearly equal to, the size of the transmission part opening of the main aperture board set. If the original image is on film, the following problem arises:
The sensitive material of the film image is made up of small particulate (diameter is 2.mu. or smaller, that have a white color) scattered as in FIG. 5A. One technical difficulty is producing particulate of sensitized material that are uniform in size, and another technical difficulty is to distribute them uniformly. Some areas have a dense distribution of these particles, while other areas have relatively few (both types of areas are usually about 10.mu.-20.mu. in size). Magnifying an image reveals this type of unevenness, therefore, resulting in graininess.
When a sharp signal is generated with a small enlargement ratio, as in the case when a full-size reading is performed, the diameter of the transmission part opening of the main aperture board set is large (as indicated by the circle .alpha.1 in FIG. 5A). Since this large diameter transmission part opening allows lights from the original image to pass through, the unevenness of the particle distribution is rectified, resulting in an excellent registration of the original film image.
When the sharp signal is generated with a large enlargement ratio, the transmission part opening of the main aperture set is small (as indicated by the circle .alpha.2 in FIG. 5B). Since this small diameter transmission part opening allows lights from the original image to pass through, the graininess passes through as well, without any rectification. The sharp signal level is low when the areas with a low density of particles are scanned as in FIG. 5C. The sharp signal level is normal when areas with a uniform distribution of particles are scanned as in FIG. 5D. The sharp signal level is high when areas with a high concentration of particles are scanned as in FIG. 5E.
When the small transmission part openings are used for scanning, as in FIG. 2A and FIG. 2B, graininess, which is an area of high concentration of particles, appears. Such graininess is read unmodified. If the outline is being enhanced, the graininess is enhanced as well.
As an alternative to the above, the large diameter transmission part opening of the main aperture board set may be used even with a high enlargement ratio setting, in order not to read the graininess. FIG. 6A shows the sharp signal, with the film image 2 scanned with the transmission part opening size equivalent to 5.mu..times.5.mu. on the original image. FIG. 6B shows the sharp signal, with the film image 2 scanned by the transmission part size equivalent to 10.mu..times.10.mu. on the original image. FIG. 6C shows the sharp signal, with the film 2 scanned by the transmission part size equivalent to 20.mu..times.20.mu. on the original image. These figures mean that the large diameter transmission part opening of the main aperture board set allows passage to the wide part of the lights from both densely and dispersively distributed areas, rectifying unevenness of particulate distribution and lowering the possibility of reading graininess.
When the large diameter transmission part opening of the main aperture board set is employed, resolution of the sharp signal is lowered because of the passage of the wide part of the lights.